1. Field of the Invention
This invention relates to optical waveguide fibers and, in particular, to methods and apparatus for testing such fibers for reflection-type discontinuities.
2. Description of the Prior Art
Optical waveguide fibers are often tested by means of an optical time-domain reflectometer (OTDR). This device sends a short pulse of laser light down a fiber and observes the small fraction of light which is scattered back towards the source. Typical pulsewidths may range from 0.5 meters (5 ns) to 2000 meters (20 .mu.s).
In practice, the fiber under test is connected to the OTDR by a relatively short length of fiber (e.g., a one kilometer length of fiber) known in the art as a "pigtail." The pigtail reduces the deadzone (non-linear region) at the start of the fiber where the OTDR does not provide reliable information. To further improve performance, an index matching oil can be used at the junction between the pigtail and the fiber.
A typical OTDR trace is shown in FIG. 1 where returned power in dBs is plotted along the y-axis and distance down the fiber is plotted along the x-axis. Various features of this trace are identified by the reference numbers 1 through 9 where the number 1 shows the reflection which occurs at the junction between the OTDR and the pigtail, the number 2 shows the trace obtained from the pigtail, the number 3 shows the last point of the pigtail and the first point of fiber under test, the number 4 shows the reflection and associated deadzone produced by the junction between the pigtail and test fiber, the number 5 shows the first point after the near-end deadzone at which trace information can be examined reliably (referred to herein as the "fiber start" or the "FS"; note that the fiber start does not correspond to the physical end of the fiber but rather to the first point from which reliable data can be obtained), the number 6 shows the fiber trace between FS and the physical end of the fiber (referred to herein as the "fiber end" or "FE"), the number 7 shows the FE, the number 8 shows the reflection which occurs at the FE, and the number 9 shows the inherent noise level of the OTDR.
Among other things, the OTDR trace is used to identify the location and magnitude of "point defects" and unexpected "reflections," i.e., reflections other than those expected at the junctions between the OTDR and the pigtail and the pigtail and the test fiber and that produced by the end of the fiber. Point defects comprise downward deviations in the OTDR trace that occur over approximately one pulsewidth. They correspond to inclusions, point discontinuities, fusion splices, and microbends in the glass making up the fiber.
Reflections comprise upward deviations or "spikes" in the OTDR trace, occurring when a point scattering site in the fiber reflects excess power back to the OTDR. They correspond to fiber breaks, contaminants in the glass making up the fiber, microbends, mechanical splices or connectors, and the like (referred to generally herein as "reflection-type discontinuities"). Reflections are typically, but not always, associated with a point defect at the same location.
An example of such an unexpected reflection (hereinafter referred to simply as a "reflection") is shown by the circled portion of the OTDR trace of FIG. 2 and in expanded form in FIG. 3. The height of this reflection relative to the straight line slope of the trace is approximately 0.12 dB, i.e., it represents a very small increase in the power reflected back to the OTDR. In this case, a point defect is also present at the reflection location (note the drop in power level before and after the reflective event in FIG. 3).
As known in the art, the height of a reflection is normally reported as its "ORL" value to eliminate the effect of pulsewidth. Specifically, the relationship between reflection height, A, and ORL value is given by: EQU ORL=B-10* LOG [(10.sup.(A/5) -1)*PW]
where B is the fiber backscattering level and PW is the OTDR pulsewidth in ns.
In the past, a method known as the "slope technique" has been used to automatically detect reflections in OTDR traces. In accordance with this method, an error array (EA) was calculated by fitting the original trace data between fiber start (FS) and fiber end (FE) with a straight line using a least squares fitting routine and then subtracting that straight line from the original trace on a point-by-point basis. The resulting error array was then examined and an alarm triggered with a rising slope exceeding a predetermined threshold was found.
In practice, the threshold level had to be set high enough so that random noise events were not falsely labeled as reflections. Accordingly, reflections having a height less than about 0.06 dB above the background noise could not be reliably found with this method for long lengths of fiber. As discussed below, with the present invention, small reflections of this type are readily found and indeed, the method works successfully down to reflections having a height of only about 0.025 dB above the background noise, i.e., the sensitivity of the present invention is more than twice on a dB scale than that of the slope technique.
The use of filters to detect relatively weak signals in noise is known in the field of digital signal processing. See, for example, John Karl, An Introduction to Digital Signal Processing, Academic Press, 1989, pages 217-225. Correlation detectors and the whitening of noise is discussed in Harry L. Van Trees, Detection, Estimation, and Modulation Theory, Part I, John Wiley & Sons, 1968, pages 246-253 and 287-293. There has been, however, no disclosure or suggestion in the art to apply such procedures to the problem of fiber inspection by an OTDR. Moreover, there has been no disclosure or suggestion that such procedures would achieve the significant improvement in reflection detection reported herein.